[rust] Genericize flexbuffer reader (#6450)

* feature/rust-tokio-bytes added feature name for tokio-bytes

* Added flexbuffer implementation, TODO: typecast to avoid recurse

* Converted codebase to utilize FlexBuffer implementation, need to resolve deserialization issues

* Added todo for lifetime issue, may use &'de [u8] for deserializer instead of current method

* Added proper &[u8] implementation

* Removed unused struct

* Added experimental fix to get_slice

* Added experimental fix to get_slice

* Avoided lifetime issues via ref structs, need to check if this hurts peformance

* Updated deserializer implementation to allow for borrowed data from Reader struct

* Fixed bug with str

* Removed unnecessary generic parameter

* Added unsafe to avoid lifetime complaints, current tests pass, need to review alternatives to unsafe

* Opinionated: Removed bytes crate as this implementation could be done in a separate crate

* Cleaned up flatbuffer

* Fixed sample / example

* Resolved PR feedback, need to resolve issues with tests

* Cleaned up FlexBuffer trait to be an auto impl

* Removed TODO

* Reverted Deserializer to only support &'de [u8]

* Cleaned up / renamed function for clarification

* Renamed FlexBuffer -> InternalBuffer for clarification on it's purpose

* Fixed issue with key bytes

* resolved issues with broken tests, confirming this is a breaking change

* Removed FIXME that's solved by splitting String and Key variants

* Implemented associated types approach

* Fixed backward slice logic

* Fixed MapReader compile error

* Added from_buffer for deserialization, removed  function since it's only needed for deserialization

* Removed dead code

* Cleaned up buffer, removed AsRef in favor of Deref

* Renamed Buffer::as_str -> Buffer::buffer_str

* Minor cleanup

* Updated documentation, need to fix tests

* Removed unnecessary &

* Removed unused lifetime

* removed unnecessary as_ref

* Minor optimization wrap-up

* resolved issue with Clone

* Added test to verify no deep-copy

* Added  for optimization

* Updated to use empty fn instead of default

* Updated comments / test name - plus the 0.3.0 version bump

* comment

rust/flexbuffers/src/reader/iter.rs

@@ -12,19 +12,20 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use super::{Reader, VectorReader};
+use crate::{Buffer, Reader, VectorReader};
 use std::iter::{DoubleEndedIterator, ExactSizeIterator, FusedIterator, Iterator};
 
 /// Iterates over a flexbuffer vector, typed vector, or map. Yields [Readers](struct.Reader.html).
 ///
 /// If any error occurs, the Reader is defaulted to a Null flexbuffer Reader.
-pub struct ReaderIterator<'de> {
-    pub(super) reader: VectorReader<'de>,
+pub struct ReaderIterator<B> {
+    pub(super) reader: VectorReader<B>,
     pub(super) front: usize,
     end: usize,
 }
-impl<'de> ReaderIterator<'de> {
-    pub(super) fn new(reader: VectorReader<'de>) -> Self {
+
+impl<B: Buffer> ReaderIterator<B> {
+    pub(super) fn new(reader: VectorReader<B>) -> Self {
         let end = reader.len();
         ReaderIterator {
             reader,
@@ -33,8 +34,9 @@ impl<'de> ReaderIterator<'de> {
         }
     }
 }
-impl<'de> Iterator for ReaderIterator<'de> {
-    type Item = Reader<'de>;
+
+impl<B: Buffer> Iterator for ReaderIterator<B> {
+    type Item = Reader<B>;
     fn next(&mut self) -> Option<Self::Item> {
         if self.front < self.end {
             let r = self.reader.idx(self.front);
@@ -49,7 +51,8 @@ impl<'de> Iterator for ReaderIterator<'de> {
         (remaining, Some(remaining))
     }
 }
-impl<'de> DoubleEndedIterator for ReaderIterator<'de> {
+
+impl<B: Buffer> DoubleEndedIterator for ReaderIterator<B> {
     fn next_back(&mut self) -> Option<Self::Item> {
         if self.front < self.end {
             self.end -= 1;
@@ -59,5 +62,6 @@ impl<'de> DoubleEndedIterator for ReaderIterator<'de> {
         }
     }
 }
-impl<'de> ExactSizeIterator for ReaderIterator<'de> {}
-impl<'de> FusedIterator for ReaderIterator<'de> {}
+
+impl<B: Buffer> ExactSizeIterator for ReaderIterator<B> {}
+impl<B: Buffer> FusedIterator for ReaderIterator<B> {}